Volcanic Activity Summary: The Alaska Volcano Observatory is rasing the Aviation Color Code to YELLOW and the Alert Level to ADVISORY at Shishaldin Volcano based on the presence of a persistent thermal anomaly in the summit crater.

Beginning in January 2009, a distinct thermal feature in the summit crater has been observed intermittently in satellite imagery. Over the past month, the intensity of this thermal feature has increased. AVO has observed this thermal anomaly several times over the past week while satellite viewing conditions were favorable. The increase in thermal emissions at Shishaldin mark a change from the normal background observed since the end of the previous eruption in 1999. Seismicity at the volcano has not changed appreciably over the time interval during which the thermal anomaly has been observed. Tiltmeters operated by UNAVCO with the National Science Foundation's Earthscope Project on the north and west flanks of the volcano show no deformation of the volcano during this time period. Satellite observations show no significant sulfur dioxide gas emissions.

AVO has received a few reports of steaming from the summit crater, which has also been observed in satellite imagery. However, this is not necessarily anomalous activity for this volcano, and the unrest observed over the past several weeks does not indicate that an eruption is imminent. AVO will continue to watch Shishaldin carefully for additional signs of increased unrest.

Remarks: Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetric cone with a base diameter of approximately 10 miles (16 km). A small summit crater typically emits a noticeable steam plume with occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, erupting atleast 28 times since 1775. Most of Shishaldin's eruptions have consisted of small ash and steam plumes, although the most recent eruption in April-May 1999 produced an ash column that reached a height of 45,000 ft above sea level.

Next Notice: A new VAN will be issued if conditions change significantly or alert levels are modified. While a VAN is in effect, regularly scheduled updates are posted athttp://www.avo.alaska.edu

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

(1) VOLCANO OBSERVATORY NOTICE FOR AVIATION (VONA)

(2) Issued:

(20090710/2214Z)

(3) Volcano:

Shishaldin (CAVW# 1101-36-)

(4) Current Color Code:

YELLOW

(5) Previous Color Code:

green

(6) Source:

Alaska Volcano Observatory

(7) Notice Number:

2009/A42

(8) Volcano Location:

N 54 deg 45 min W 163 deg 58 min

(9) Area:

Aleutians Alaska

(10) Summit Elevation:

9373 ft (2857 m)

(11) Volcanic Activity Summary:

The Alaska Volcano Observatory is rasing the Aviation Color Code to YELLOW and the Alert Level to ADVISORY at Shishaldin Volcano based on the presence of a persistent thermal anomaly in the summit crater.

Beginning in January 2009, a distinct thermal feature in the summit crater has been observed intermittently in satellite imagery. Over the past month, the intensity of this thermal feature has increased. AVO has observed this thermal anomaly several times over the past week while satellite viewing conditions were favorable. The increase in thermal emissions at Shishaldin mark a change from the normal background observed since the end of the previous eruption in 1999. Seismicity at the volcano has not changed appreciably over the time interval during which the thermal anomaly has been observed. Tiltmeters operated by UNAVCO with the National Science Foundation's Earthscope Project on the north and west flanks of the volcano show no deformation of the volcano during this time period. Satellite observations show no significant sulfur dioxide gas emissions.

AVO has received a few reports of steaming from the summit crater, which has also been observed in satellite imagery. However, this is not necessarily anomalous activity for this volcano, and the unrest observed over the past several weeks does not indicate that an eruption is imminent. AVO will continue to watch Shishaldin carefully for additional signs of increased unrest.

(12) Volcanic cloud height:

Unknown

(13) Other volcanic cloud information:

Unknown

(14) Remarks:

Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetric cone with a base diameter of approximately 10 miles (16 km). A small summit crater typically emits a noticeable steam plume with occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, erupting atleast 28 times since 1775. Most of Shishaldin's eruptions have consisted of small ash and steam plumes, although the most recent eruption in April-May 1999 produced an ash column that reached a height of 45,000 ft above sea level.

A new VONA will be issued if conditions change significantly or alert levels are modified. While a VONA is in effect, regularly scheduled updates are posted athttp://www.avo.alaska.edu

VOLCANO ALERT LEVELS

NORMALVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORYVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

WATCHVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway but poses limited hazards.

WARNINGHazardous eruption is imminent, underway, or suspected.

AVIATION COLOR CODES

GREENVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOWVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

ORANGEVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

REDEruption is imminent with significant emission of volcanic ash into the atmosphere likely OR eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].